|Two Flow Measurements
Figure 7. This approach provides faster response than that with a temperature controller.
The steady-state separation model assumes the total material balance around the condenser and reflux drum is enforced at all times, i.e.:
V = L + D
where V is overhead vapor flow, D is distillate flow and L is reflux flow. The reflux drum level controller is responsible for closing this material balance. It will close the material balance over the long term — but not in the short term.
For the control configuration shown in Figure 6, the performance of the temperature controller depends on changes in distillate flow quickly translating into changes in reflux flow, which is the responsibility of the level controller. Suppose the level controller is on manual, giving a constant reflux flow. Then, the only effect of increasing distillate flow is to drain liquid from the reflux drum; changes in distillate flow don't affect overhead composition or the temperature of the control stage in the upper section.
The term "cascade" as normally understood doesn't apply to the relationship between the upper-stage temperature controller and the drum level controller. However, two attributes of cascade configurations do come into play in the control scheme in Figure 6:
1. The temperature controller totally depends on the level controller (changes in distillate flow must be translated to changes in reflux flow).
2. Reflux flow must change rapidly with respect to the dynamics of the temperature loop.
The dynamics of the drum level loop preferably should be about five times faster than those of the temperature controller. How rapidly the level controller can respond depends on the size of the reflux drum and how much noise is present on the drum level measurement.
Alternatives to the configuration in Figure 6 deserve consideration. Using two flow measurements (distillate flow and one more), you can provide logic that translates a change in distillate flow into an equal and opposite change in reflux flow. The dynamics will be typical of flow loops and, thus, far faster than those of the temperature loop.
Figure 7 presents one approach. Its key attributes are:
• The output of the drum level controller is considered the target for the total flow leaving the reflux drum.
• The measured value of distillate flow is subtracted from the total to give the set point for reflux flow.
This configuration is a level-to-flow cascade with a computed flow set point. You can impose the minimum reflux flow as a lower set point limit on the reflux flow controller.
Figure 8 illustrates an alternative approach. Its key attributes are:
• The output of the drum level controller is the set point for a flow controller whose process variable (PV) is the computed total flow from the reflux drum.
• The total flow from the reflux drum is computed by summing the measured values for reflux and distillate flows.
The configuration is a simple level-to-flow cascade with a computed value for the flow.
Figure 9 shows yet another approach. Its key attributes are: